Serious problems have been encountered in the operation of truck, tank and helicopter engines in dusty, sandy or ocean environments where particularly corrosive contaminants are prevelant. If not filtered, such contaminants can corrode or wear operating parts excessively, necessitating more frequent maintenance and significantly reducing the operating life of the engine.
Various air filters have been developed for such applications, utilizing a variety of air cleaning techniques. For instance, inertial separation, also known as centrifical separation, has been found to be an efficient means to remove large amounts of dust or sand from large airstreams. Other contaminants, however, are not easily removed by inertial separation and must be removed by a barrier filter element or like apparatus.
Due to the nature of the contaminants removed by inertial separators and the nature of their operation they have a relatively long operating life between cleanings. This makes them particularly well adapted for use in demanding military environments where relatively long operating life between maintainence is always important and sometimes essential. On the other hand, barrier filter elements generally have a much shorter operating life between cleanings, especially when used in dusty environments. Such filters are, however, quite efficient in removing finer particulate matter and other contaminants such that numerous mechanisms have been developed to clean the filter while it is operating and thereby extend the interval between maintainence.
It has been recognized that combining inertial separation for pre-cleaning with self-cleaning barrier filter techniques produces an efficient air cleaner with a relatively long service life. An example of such a design is shown in U.S. Pat. No. 4,303,423 to Camplin et al. That filter, however, and others like it, is susceptible to reintrainment of particulate contaminants dislodged from the filter during cleaning because the cleansing reverse air flow tends to interrupt the operation of the pre-cleaner swirl, such that dislodged contaminants are reintroduced to the cleaning medium instead of being scavenged away. Also, that filter has no bypass capability, which as U.S. Pat. No. 3,483,676 to Sargisson teaches is highly desireable such that when necessary unimpeded air intake may be provided to attain maximum performance of the engine.